It is legal and can be done. Can be fairly costly depending on the origins of your van. Some vans enter the country as cars and are then converted, but some enter as vans and these are the ones that are harder to convert. Someone in the know should be able to tell you with a quick inspection on VIN.

Sell it and buy or import a car version, unless it's about 3 years old or less it won't be worth while, you will be talking about the guts of €5 or 6k to do it right as you will have pay VRT and have then buy and fit seats, belts with new anchor bolts, rear window glass with fittings. When you come to sell the car you will likely have little interest as the conversion will put people off.

If it was converting the other way it can be alot easier. There's no way it would be economical to convert it to passneger unless it origionally was a passenger. Just sell it and buy a passenger. It'll the ALOT less hassle.

You would be liable for VRT, and if you convert the vehicle and don't sort the VRT out, it can be seized, with a penalty and the VRT to be paid before it will be released. Possible court appearance?

+1

Jaysus i could see some one going from passenger to commercial, but commercial to passenger? I really wouldn't bother, just sell it and buy a passenger, it'll be a alot less hassle, paper work, etc. etc. Either way you will have to pay VRT, which will all the costs and time added up could cost you more in the long run.